Mixer circuits are generally used to convert an input signal at one frequency to another frequency according to a local oscillator (LO) frequency. For down conversion applications, mixers convert higher input frequency e.g. radio frequency (RF) to a lower output frequency e.g. intermediate frequency (IF), and for up conversion applications, the input frequency will be a lower frequency IF signal and the output will be higher frequency RF signal. The local oscillator (LO) frequency or signal is also known as the reference frequency or signal and the higher, radio frequency (RF) frequency or signal is also known as the data frequency signal. Those frequency conversions are realized through the non-linear operations of the mixer cores embedded in the mixer circuits. Along with the desired mixed output signal, the mixer typically generates other intermodulation products and port-to-port leakages that are preferably suppressed. Mixer designs can be categorized into one of three topologies: single-ended mixer, single-balanced mixer and double balanced mixer. The double balanced topology employs balanced mixing core structures, such as ring, bridge or star forms, and the mixer cores are usually driven differentially by two baluns. This balanced differential drive provides high isolation from port to port, high rejection of even-mode harmonics and intermodulation products and high power handling (thus, good dynamic range). Therefore, double-balanced topology is usually the preferred choice for high-performance applications. Broadband mixing core topologies have been previously proposed and studied in papers, such as Fong-Cheng Chang, et al., “A 4-41 GHz Single Balanced Distributed Mixer Using GaAs pHEMT Technology”, IEEE Microwave and Wireless Components Letters, Vol. 17, No. 2. February 2007 and Amin Q. Safarian, et al., “Design and Analysis of an Ultrawide-Band Distributed CMOS Mixer”, IEEE Trans. On Very Large Scale Integration (VLSI) Systems, Vol. 13, No. 5, May, 2005, U.S. Pat. No. 4,125,810 to Pavio, U.S. Pat. No. 4,224,572 to Will, U.S. Pat. No. 4,355,421 to Seely, U.S. Pub. No. US2005/0064840A1 to Heydari, U.S. Pat. No. 6,850,575B1 to Ahmed, U.S. Pat. No. 6,993,312 B1 to Salib, and U.S. Pat. No. 7,013,122 B2 to Gamliel. Among them the balanced distributed topologies are one of the most promising approaches to achieve ultra-wide bandwidth without degradation of other mixer performance. However, all of the previous double-balanced distributed mixers are either active (or Gilbert-cell based) mixers that require active bias currents or single-balanced FET based mixers. Both topologies suffer from poor power handling capability and low linearity. In addition, the active distributed mixers have poor noise performance. And single-balanced distributed FET mixers suffer from low port-to-port isolation compared with double-balanced mixers. In summary, existing passive distributed-mixer designs are either single-balanced or totally un-balanced which lead to poor isolation between ports. The active double balanced distributed mixers can provide good isolation using balanced topologies but suffer from poor noise performance, which greatly limits the active mixer's dynamic range.
One prior art approach, U.S. Pat. No. 5,854,974 uses a compensation inductor between the differential reference (LO) nodes of a diode ring to resonate with the capacitive reactance of the diode ring at the reference (LO) frequency. This inductor is located across the differential output of the reference (LO) balun in parallel with the diode ring and it will cancel out the capacitance exhibited by diode ring at reference (LO) frequency. In this way, the linearity is improved with reduced 3rd order harmonics at reference (LO) frequency. Also disclosed was the use of two inductors with two ring mixer cores where each inductor is connected across the reference (LO) differential nodes of each ring to resonate with each ring's capacitance at reference (LO) frequency. However, because of the resonance nature of this design, this approach only works for a narrow reference (LO) frequency bandwidth and therefore it is not suitable for broadband designs.